1. Field of Invention
This invention relates to a strain sensor, specifically to a sensor that can measure axial and bending strain.
2. Background of the Invention
Variable capacitors play a fundamental role in high-frequency and radio-frequency (RF) circuits. In the last few years, MEMS variable capacitors have drawn considerable interest due to their superior electrical characteristics, size and cost of manufacture.
While variable capacitors using MEMS technology can be readily implemented in standard semiconductor devices for applications in aerospace, consumer electronics and communications systems, researchers have attempted to provide application to medical systems or diagnostics. Modern medical science has emerged with a need to monitor physiological functions (i.e. intravascular pressure, intraocular pressure, etc.). A variety of these monitoring devices require that their tasks be performed wirelessly and implanted for indefinite terms to allow for patient mobility, continuance of daily activities and avoidance of costly surgeries to remove the systems after utilization is complete.
An application of a MEMS bending and axial capacitive sensor is to monitor strain changes of spinal instrumentation implanted during spinal fusion surgical procedures to assist orthopaedic surgeons with evaluation of fusion progression. The current method to assess fusion is the evaluation of radiographic images. However, image obstructions often prevent a clear determination if fusion has occurred. A strain sensor could be incorporated with a battery-less implantable telemetry system and enclosed in a hermetically sealed package. After attachment to the spinal instrumentation, the sensor can vary its capacitance output due to small changes in strain by the instrumentation as fusion occurs, thereby giving objective data to the orthopaedic surgeons of whether fusion is occurring and potentially avoiding costly exploratory surgery.
Existing strain sensors are used to indicate the amount and the type of deformation (i.e. elongation or compression) of materials. These can be used to indicate the state of a material, predict material behavior or gather material properties. These types of sensors can gather information in a variety of manners including changes in resistance and capacitance. However, there has not been a sensor available that can measure bending and axial strain in a capacitive manner.
Inventors have developed sensors in attempts to measure bending strain in a capacitive manner. U.S. Pat. No. 5,827,980 to Doemens (1998) has developed a dual comb structure; however, the orientation of the device is situated at 45 degrees and is not meant to observe bending strain. Additionally by Doemens, U.S. Pat. No. 5,750,904 (1998) the inventor has proposed a dual pair of comb structures that primarily measure axial forces or extension forces. However, bending is not the primary method of actuation which would cause the comb structures to move vertically causing a change in the overlapping surface area.
U.S. Pat. No. 6,606,913 to Gianchandani (2003) has disclosed a complex array of elevated small comb structures or tines with vertical sidewalls. While undergoing axial strain, the tines will change their overlapping surface area, which can be correlated to change in capacitance and strain. However, due to the attachment method of securing both ends of the comb structures to the substrate, the sensor would not be able to actuate while undergoing bending strain. A similar case is made with U.S. Pat. No. 7,035,083 to Lin (2006), where the attachment method of the tines will not allow vertical displacement allowing a change in the overlapping surface area of the tines.
Other U.S. Pat. Nos. 4,188,651 (1980), 4,941,363 (1990), 5,610,528 (1997), 6,266,226 (2001) and 6,532,824 (2003) identify capacitive comb structures comprised on a thin film. However, the lack of vertical dimension or overlapping surface area can not be used to determine the amount of bending strain present in a deformed substrate.
Comb structures are prolific throughout the MEMS environment; however, very few are actuated by attachment via a substrate. Most examples are actuated by the electrostatic means of applying a voltage potential to the independent comb structures as noted by Wu, U.S. Pat. No. 7,085,122 (2006), Lin, U.S. Pat. No. 5,537,083 (1996), Gang, U.S. Pat. No. 5,918,280 (1999), Muenzel, U.S. Pat. No. 5,723,353 (1998) and Nguyen, U.S. Pat. Nos. 6,236,281 (2001), 5,955,932 (1999), 5,839,062 (1998), 5,491,604 (1996).